Splitter and combiner for multiple data rate communication system

ABSTRACT

Complementary band-splitter and band-combiner devices and methods of operation of both are disclosed. Embodiments of the invention may allow wideband and narrowband functions and services to operate seamlessly by band-splitting wideband data into low-band data and high-band data. Narrowband data may comprise only low-band data. Wideband services may operate on both low-band and high-band data, while narrowband services may operate only on low-band data. Another embodiment of the present invention may include machine-readable storage having stored thereon a computer program having a plurality of code sections executable by a machine for causing the machine to perform the foregoing.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

[0001] The applicants claim priority based on provisional applicationNo. 60/414,491, “Splitter and Combiner for Multiple Data RateCommunication System”, filed Sep. 27, 2002, the complete subject matterof which is incorporated herein by reference in its entirety.

[0002] This application is also related to the following co-pendingapplications, each of which are herein incorporated by reference: Ser.No. Docket No. Title Filed Inventors 60/414,059 14057US01 Multiple DataRate Communication Sep. 27, 2002 LeBlanc System Houghton Cheung60/414,460 14061US01 Dual Rate Single Band Communication Sep. 27, 2002LeBlanc System Houghton Cheung 60/414,493 14064US01 Switchboard forMultiple Data Rate Sep. 27, 2002 LeBlanc Communication System HoughtonCheung 60/414,492 14062US01 Method and System for an Adaptive Sep. 27,2002 LeBlanc Multimode Media Queue Houghton Cheung

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0003] [Not Applicable][MICROFICHE/COPYRIGHT REFERENCE]

[0004] [Not Applicable]

BACKGROUND OF THE INVENTION

[0005] Traditional voice telephony products are band-limited to 4 kHzbandwidth with 8 kHz sampling. These products include the telephone,data modems, and fax machines. Newer products aiming to achieve highervoice quality have doubled the sampling rate to 16 kHz to encompass alarger 8 kHz bandwidth, which is also known as “wideband” capable. Thesoftware implications of doubling the sampling rate are significant.Doubling the sampling rate not only requires doubling the processingcycles, but nearly doubling the memory used to store the data. Inaddition, software supporting wideband capabilities must not precludesupport for legacy 4 kHz band-limited functionality.

[0006] Doubling memory and processor cycles requirements is expensivebecause the memory and processing power footprints of digital signalprocessors (DSPs) are generally small. Implementing wideband supportthus requires creativeness to optimize both memory and cycles.

[0007] Additionally, much of the software providing various functionsand services, such as echo cancellation, dual-tone multi-frequency(DTMF) detection and generation, and call discrimination (between voiceand facsimile transmission, for example), are written for onlynarrowband signals. Either new software must be written for widebandsignals, or the wideband signal must be down-sampled. Where the softwareis modified, the software should also be capable of integration withpreexisting narrowband devices. Providing software for operation withboth narrowband and wideband devices is complex and costly.

[0008] Accordingly, there is a need for splitting functionality forsplitting a wideband data stream into a low-band data stream and ahigh-band data stream, as well as combining functionality for combininga low-band data stream with a high-band data stream to produce awideband data stream.

[0009] Further limitations and disadvantages of conventional andtraditional approaches will become apparent to one of skill in the art,through comparison of such systems with aspects of the present inventionas set forth in the remainder of the present application with referenceto the drawings.

BRIEF SUMMARY OF THE INVENTION

[0010] Seamless wideband support is afforded by utilizing band-splitdata streams. In an illustrative embodiment of the present invention,the 8 kHz bandwidth is divided into a low band, with approximately 0-4kHz bandwidth, and a high band, with approximately 4-8 kHz bandwidth.Narrowband functions and services operate on the low band, whilewideband functions and services operate on both low and high bands.Splitting functionality splits a wideband data stream into a low bandand a high band. Combining functionality combines a low-band data streamwith a high-band data stream to produce a wideband data stream.

[0011] An aspect of the present invention may be seen in a band splittercomprising at least one filter for modifying wideband data, producinglow-band data, a delay element for delaying the wideband data; and adevice for removing the low-band data from the delayed wideband data,producing high-band data. The wideband data may represent spectralcomponents from a predetermined lower frequency to a predetermined upperfrequency, where the predetermined lower frequency may be approximately0 Hz and the predetermined upper frequency may be approximately 8 kHz.The low-band data may represent spectral components less than apredetermined frequency, and the high-band data may represent spectralcomponents greater than the predetermined frequency, where thepredetermined frequency may be approximately 4 kHz. In addition, thespectral mask of the low-band data may meet the spectral mask of G.712.The at least one filter may further comprise a down sampler for reducingthe sampling rate of the low-band data.

[0012] In another embodiment, the band splitter may comprise a firstfilter for filtering wideband data, a down-sampler for down-sampling theoutput of the first filter producing low-band data, an up-sampler forup-sampling the low-band data, a second filter for filtering theup-sampled low-band data, a delay element to delay the wideband data,and a device for removing the output of the second filter from thedelayed wideband data, producing high-band data. The wideband data mayrepresent spectral components from a predetermined lower frequency to apredetermined upper frequency, where the predetermined lower frequencymay be approximately 0 Hz and the predetermined upper frequency may beapproximately 8 kHz. The low-band data may represent spectral componentsless than a predetermined frequency, and the high-band data mayrepresent spectral components greater than the predetermined frequency,where the predetermined frequency may be approximately 4 kHz. Thespectral mask of the low-band data may meet the spectral mask of G.712.

[0013] A further embodiment of the present invention can be seen in aband combiner comprising an up-sampler to up-sample low-band data, afilter for filtering the output of the up-sampler, and an adder forcombining the output of the filter and high-band data, producingwideband data. The wideband data may represent spectral components froma predetermined lower frequency to a predetermined upper frequency,where the predetermined lower frequency may be approximately 0 Hz. andthe predetermined upper frequency may be approximately 8 kHz. Thespectral mask of the low-band data may meet the spectral mask of G.712.

[0014] Another aspect of the present invention relates to a method ofsplitting wideband data into low-band data and high-band data, themethod comprising filtering the wideband data to produce low-band data,delaying the wideband data, and removing the low-band data from thedelayed wideband data to produce high-band data. In such an embodiment,the wideband data may represent spectral components from a predeterminedlower frequency to a predetermined upper frequency, where thepredetermined lower frequency is approximately 0 Hz, and thepredetermined upper frequency may be approximately 8 kHz. The low-banddata may represent spectral components less than a predeterminedfrequency, and the high-band data may represent spectral componentsgreater than the predetermined frequency, where the predeterminedfrequency may be approximately 4 kHz. In addition, the spectral mask ofthe low-band data may meet the spectral mask of G.712.

[0015] Yet another embodiment in accordance with the present inventionis a method of splitting wideband data into low-band data and high-banddata, the method comprising filtering the wideband data, down-samplingthe filtered wideband data to produce low-band data, up-sampling thelow-band data, filtering the up-sampled low-band data, delaying thewideband data, and removing the filtered up-sampled low-band data fromthe delayed wideband data to produce high-band data. The wideband datain such an embodiment may represent spectral components from apredetermined lower frequency to a predetermined upper frequency, wherethe predetermined lower frequency is approximately 0 Hz, and thepredetermined upper frequency may be approximately 8 kHz. The low-banddata may represent spectral components less than a predeterminedfrequency, and the high-band data may represent spectral componentsgreater than the predetermined frequency, where the predeterminedfrequency may be approximately 4 kHz. In addition, the spectral mask ofthe low-band data may meet the spectral mask of G.712

[0016] A further embodiment of the present invention relates to a methodof combining low-band data and high-band data to produce wideband data,the method comprising up-sampling the low-band data, filtering theup-sampled low-band data, and adding the filtered up-sampled low-banddata to the high-band data to produce wideband data. The wideband datamay represent spectral components from a predetermined lower frequencyto a predetermined upper frequency, where the predetermined lowerfrequency may be approximately 0 Hz and the predetermined upperfrequency may be approximately 8 kHz. The low-band data may representspectral components less than a predetermined frequency, and thehigh-band data represents spectral components greater the predeterminedfrequency, where the predetermined frequency may be approximately 4 kHz.The spectral mask of the low-band data may meet the spectral mask ofG.712.

[0017] A further embodiment of the present invention may includemachine-readable storage, having stored thereon a computer programhaving a plurality of code sections executable by a machine for causingthe machine to perform the foregoing.

[0018] These and other advantages, aspects, and novel features of thepresent invention, as well as details of illustrated embodiments,thereof, will be more fully understood from the following descriptionand drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

[0019]FIG. 1 is a block diagram of an exemplary communication systemwherein the present invention can be practiced.

[0020]FIG. 2 is a signal flow diagram for a split-band architecture inaccordance with an embodiment of the present invention.

[0021]FIG. 3 is a system block diagram of a signal processing systemoperating in a voice mode in accordance with an illustrative embodimentof the present invention.

[0022]FIG. 4 is a block diagram of a band splitter in accordance with anillustrative embodiment of the present invention.

[0023]FIG. 5 is a block diagram of a band combiner in accordance with anillustrative embodiment of the present invention.

[0024]FIG. 6 is a block diagram of an exemplary terminal in whichaspects of the present invention may be practiced.

DETAILED DESCRIPTION OF THE INVENTION

[0025] Referring now to FIG. 1, there is illustrated a block diagram ofan exemplary voice-over-packet (VoP) network 100 wherein the presentinvention can be practiced. The VoP network 100 comprises a packetnetwork 105 and a plurality of terminals 110. The terminals 110 arecapable of receiving user input. The user input can comprise, forexample, the user's voice, video, or a document for facsimiletransmission. The VoP network 100 supports various communicationsessions between terminals 110 which simulate voice calls and/orfacsimile transmissions over a switched telephone network.

[0026] The terminals 110 are equipped to convert the user input into anelectronic signal, digitize the electronic signal, and packetize thedigital samples. The sampling rate for digitizing the electronic signalcan be either 8 kHz (narrowband) sampling, or 16 kHz (wideband)sampling. Accordingly, narrowband sampling is bandwidth limited to 4 kHzwhile wideband sampling is bandwidth limited to 8 kHz.

[0027] The VoP network 100 provides various functions and services,including dual-tone multi-frequency (DTMF) generation and detection, andcall discrimination between voice and facsimile, by means of a VirtualHausware Device (VHD) and a Physical Device Driver (PXD). The foregoingservices are implemented by software modules and utilize narrowbanddigitized samples for inputs. For terminals 110 with narrowbandsampling, the digitized samples are provided directly to the softwaremodules. For terminals 110 with wideband sampling, the 8 kHz bandwidthis split into a high-band data stream and a G.712 compliant low-banddata stream. The software modules requiring narrowband digitized samplesoperate on the low-band data, while software modules requiring widebanddigitized samples operate on both the high-band data and the low-banddata.

[0028] The low-band data is stored as 8 kHz sampled data, while thehigh-band data is stored as 16 kHz sampled data. In one embodiment, bothbands are not stored symmetrically as 8 kHz sampled data because the 8kHz bandwidth is not split symmetrically down the center. This designincurs a memory cost in return for voice quality and G.712 compliance.Alternatively, if aliasing may be ignored, the 8 kHz bandwidth may besplit symmetrically with both low-band data and high-band data stored as8 kHz sampled data. This alternative avoids the increased memoryrequirement, but at the cost of voice quality. Both symmetric andasymmetric split-band architectures are similar in implementation exceptfor the sampling rate of the media streams. In some designs, one may bemore desirable. In other designs, the reverse may be true. The optimalchoice depends on an acceptable memory versus performance trade-off.

[0029] The split-band approach enables straightforward support fornarrowband and wideband services because narrowband services areincognizant of the wideband support. Narrowband services operate on the8 kHz-sampled stream of data (i.e., the low-band data). Generally, onlywideband services understand and operate on both bands.

[0030] Referring now to FIG. 2, there is illustrated a signal flowdiagram of a split-band architecture 200 in accordance with anembodiment of the present invention. The split-band architecture 200includes a Virtual Hausware Driver (VHD) 205, a switchboard 210, aphysical device driver (PXD) 215, an interpolator 220, and a decimator225.

[0031] The PXD 215 represents an interface for receiving the inputsignal from the user and performs various functions, such as echocancellation. The order of the PXD 215 functions maintains continuityand consistency of the data flow. The top of the PXD 215 is at theswitchboard 210 interface. The bottom of the PXD 215 is at theinterpolator 220 and decimator 225 interface. For wideband operation,the band splitter/combiner PXD 215 function may be located as follows.On the switchboard 210 side of this PXD 215 function is split-band data.On the other side is single-band data. PXD 215 functions that operate onsingle-band data, like side-tone or high-pass PXD 215 functions, areordered below the band splitter/combiner PXD 215 function. Other PXD 215functions that operate on split-band data are ordered above it.

[0032] The VHD 205 is a logical interface to destination terminal 110via the packet network 105 and performs functions such as dual tonemulti-frequency (DTMF) detection and generation, and call discrimination(CDIS). During a communication (voice, video, fax) between terminals,each terminal 110 associates a VHD 205 with each of the terminal(s) 110with which it is communicating. For example, during a voice-over-packet(VoP) network call between two terminals 110, each terminal 110associates a VHD 205 with the other terminal 110. The switchboard 210associates the VHD 205 and the PXD 215 in a manner that will bedescribed below.

[0033] A wideband system may contain a mix of narrowband and widebandVHDs 205 and PXDs 215. A difference between narrowband and widebanddevice drivers is their ingress and egress sample buffer interface. Awideband VHD 205 or PXD 215 has useful data at its high and low-bandsample buffer interfaces and can include both narrowband and widebandservices and functions. A narrowband VHD 205 or PXD 215 has useful dataat its low-band sample buffer interface and no data at its high-bandsample buffer interface. The switchboard interfaces with narrowband andwideband VHDs 205 and PXDs 215 through their high and low-band samplebuffer interfaces. The switchboard 210 is incognizant of the wideband ornarrowband nature of the device drivers. The switchboard 210 reads andwrites data through the sample buffer interfaces. The high and low-bandsample buffer interfaces may provide data at any arbitrary samplingrate. In an embodiment of the present invention, the low-band samplebuffer interface provides data sampled at 8 kHz and the high-band samplebuffer interface provides data sampled at 16 kHz. Additionally, a VHD205 can be dynamically changed between wideband and narrowband and viceversa.

[0034] The VHD 205 and PXD 215 driver structures may include sample rateinformation to identify the sampling rates of the high and low-banddata. The information may be part of the interface structure that theswitchboard understands and may contain a buffer pointer and anenumeration constant or the number of samples to indicate the samplerate.

[0035] The split-band architecture 200 is also characterized by aningress path and an egress path, wherein the ingress path transmits userinputs to the packet network, and wherein the egress path receivespackets from the packet network 105. The ingress path and the egresspath can either operate in a wideband support mode, or a narrowbandmode. Additionally, although the illustrated ingress path and egresspath are both operating in the wideband support mode, the ingress pathand the egress path are not required to operate in the same mode. Forexample, the ingress path can operate in the wideband support mode,while the egress path operates in the narrowband mode. The ingress pathcomprises the decimator 225, band splitter 230, echo canceller 235,switchboard 210, and services including but not limited to DTMF detector240, CDIS 245, and packet voice engine (PVE) 255 comprising a combiner250 and an encoder algorithm 260. The switchboard 210 can comprise theswitchboard described in provisional patent application Ser. No.60/414,493, “Switchboard for Multiple Data Rate Communication System”,which is incorporated by reference in its entirety.

[0036] In a wideband device, the decimator 225 receives the user inputsand provides 16 kHz sampled data for an 8 kHz band-limited signal. The16 kHz sampled data is received by the band splitter 230. The bandsplitter 230 splits the 8 kHz bandwidth into low-band data (L) andhigh-band data (H). The low-band data, L, and high-band data, H, aretransmitted through the echo canceller 235, and switchboard 210 to theVHD 205 associated with the destination terminal 110.

[0037] The VHD 205 receives the low-band data, L, and high-band data, H.In some cases, the DTMF detector 240 may be designed for operation ononly narrowband digitized samples, and only the low-band data is passedto DTMF detector 240. Similarly, where CDIS 245 is designed foroperation on only narrowband digitized samples, only the low-band datais provided to CDIS 245, which distinguishes a voice call from afacsimile transmission. The low-band data, L, and high-band data, H, arecombined at a combiner 250 in PVE 255.

[0038] The PVE 255 is responsible for issuing media queue mode changecommands consistent with the active encoder and decoder. The mediaqueues can comprise, for example, the media queues described inprovisional patent application Ser. No. 60/414,492, “Method and Systemfor an Adaptive Multimode Media Queue”, which is incorporated herein byreference in its entirety.

[0039] The PVE 255 ingress thread receives raw samples. The raw samplesinclude both low and high-band data. However, to save memory onlylow-band data is forwarded when the VHD 205 is operating in narrowbandmode. Both low and high-band data are combined and forwarded whenoperating in wideband mode.

[0040] At PVE 255, encoder 260 packetizes the combined signal fortransmission over the packet network 105. The encoder 260 can comprise,for example, the BroadVoice 32 Encoder made by Broadcom, Inc.

[0041] The egress path comprises decoder 263, band splitter 264, CDIS266, DTMF generator 269, switchboard 210, echo canceller 235, bandcombiner 272, and interpolator 220. The egress queue receives datapackets from the packet network 105 at the decoder 263. The decoder 263can comprise the BroadVoice 32 Decoder made by Broadcom, Inc. Thedecoder 263 decodes data packets received from the packet network 105and provides 16 kHz sampled data. The 16 kHz sampled data is provided toband splitter 264 which separates low-band data, L1, from high-banddata, H1. Again, in one embodiment, where CDIS 266 and DTMF generator269 require narrowband digitized samples, only the low-band data is usedby CDIS 266 and the DTMF generator 269.

[0042] The DTMF generator 269 generates DTMF tones if detected from thesending terminal 110. These tones are written to the low-band data, L1.The low-band data, L1, and high-band data, H1, are received by theswitchboard 210. The switchboard 210 provides the low-band data, L1, andhigh-band data, H1, to the PXD 215. The low-band data, L1, and high-banddata, H1, are passed through the echo canceller 235 and provided to theband combiner 272 which combines the low-band data, L1, and high-banddata, H1. The combined low-band data, L1, and high-band data, H1, areprovided to interpolator 220. The interpolator 220 provides 16 kHzsampled data.

[0043] The services invoked by the network VHD in the voice mode and theassociated PXD are shown schematically in FIG. 3. In the describedexemplary embodiment, the PXD 60 provides two-way communication with atelephone or a circuit-switched network, such as a PSTN line (e.g. DSO)carrying a 64 kb/s pulse code modulated (PCM) signal, i.e., digitalvoice samples.

[0044] The incoming PCM signal 60 a is initially processed by the PXD 60to remove far-end echoes that might otherwise be transmitted back to thefar-end user. As the name implies, echoes in telephone systems are thereturn of the talker's voice resulting from the operation of the hybridwith its two-four wire conversion. If there is low end-to-end delay,echo from the far end is equivalent to side-tone (echo from thenear-end), and therefore, not a problem. Side-tone gives users feedbackas to how loudly they are talking, and indeed, without side-tone, userstend to talk too loudly. However, far-end echo delays of more than about10 to 30 msec significantly degrade the voice quality and are a majorannoyance to the user.

[0045] An echo canceller 70 is used to remove echoes from far-end speechpresent on the incoming PCM signal 60 a before routing the incoming PCMsignal 60 a back to the far-end user. The echo canceller 70 samples anoutgoing PCM signal 60 b from the far-end user, filters it, and combinesit with the incoming PCM signal 60 a. Preferably, the echo canceller 70is followed by a non-linear processor (NLP) 72 which may mute thedigital voice samples when far-end speech is detected in the absence ofnear-end speech. The echo canceller 70 may also inject comfort noisewhich in the absence of near-end speech may be roughly at the same levelas the true background noise or at a fixed level.

[0046] After echo cancellation, the power level of the digital voicesamples is normalized by an automatic gain control (AGC) 74 to ensurethat the conversation is of an acceptable loudness. Alternatively, theAGC can be performed before the echo canceller 70. However, thisapproach would entail a more complex design because the gain would alsohave to be applied to the sampled outgoing PCM signal 60 b. In thedescribed exemplary embodiment, the AGC 74 is designed to adapt slowly,although it should adapt fairly quickly if overflow or clipping isdetected. The AGC adaptation should be held fixed if the NLP 72 isactivated.

[0047] After AGC, the digital voice samples are placed in the mediaqueue 66 in the network VHD 62 via the switchboard 32′. In the voicemode, the network VHD 62 invokes three services, namely calldiscrimination, packet voice exchange, and packet tone exchange. Thecall discriminator 68 analyzes the digital voice samples from the mediaqueue to determine whether a 2100 Hz tone, a 1100 Hz tone or V.21modulated HDLC flags are present. If either tone or HDLC flags aredetected, the voice mode services are terminated and the appropriateservice for fax or modem operation is initiated. In the absence of a2100 Hz tone, a 1100 Hz tone, or HDLC flags, the digital voice samplesare coupled to the encoder system which includes a voice encoder 82, avoice activity detector (VAD) 80, a comfort noise estimator 81, a DTMFdetector 76, a call progress tone detector 77 and a packetization engine78.

[0048] Typical telephone conversations have as much as sixty percentsilence or inactive content. Therefore, high bandwidth gains can berealized if digital voice samples are suppressed during these periods. AVAD 80, operating under the packet voice exchange, is used to accomplishthis function. The VAD 80 attempts to detect digital voice samples thatdo not contain active speech. During periods of inactive speech, thecomfort noise estimator 81 couples silence identifier (SID) packets to apacketization engine 78. The SID packets contain voice parameters thatallow the reconstruction of the background noise at the far end.

[0049] From a system point of view, the VAD 80 may be sensitive to thechange in the NLP 72. For example, when the NLP 72 is activated, the VAD80 may immediately declare that voice is inactive. In that instance, theVAD 80 may have problems tracking the true background noise level. Ifthe echo canceller 70 generates comfort noise during periods of inactivespeech, it may have a different spectral characteristic from the truebackground noise. The VAD 80 may detect a change in noise character whenthe NLP 72 is activated (or deactivated) and declare the comfort noiseas active speech. For these reasons, the VAD 80 should generally bedisabled when the NLP 72 is activated. This is accomplished by a “NLPon” message 72 a passed from the NLP 72 to the VAD 80.

[0050] The voice encoder 82, operating under the packet voice exchange,can be a straight 16-bit PCM encoder or any voice encoder which supportsone or more of the standards promulgated by ITU. The encoded digitalvoice samples are formatted into a voice packet (or packets) by thepacketization engine 78. These voice packets are formatted according toan applications protocol and sent to the host (not shown). The voiceencoder 82 is invoked only when digital voice samples with speech aredetected by the VAD 80. Since the packetization interval may be amultiple of an encoding interval, both the VAD 80 and the packetizationengine 78 should cooperate to decide whether or not the voice encoder 82is invoked. For example, if the packetization interval is 10 msec andthe encoder interval is 5 msec (a frame of digital voice samples is 5ms), then a frame containing active speech should cause the subsequentframe to be placed in the 10 ms packet regardless of the VAD stateduring that subsequent frame. This interaction can be accomplished bythe VAD 80 passing an “active” flag 80 a to the packetization engine 78,and the packetization engine 78 controlling whether or not the voiceencoder 82 is invoked.

[0051] In the described exemplary embodiment, the VAD 80 is appliedafter the AGC 74. This approach provides optimal flexibility becauseboth the VAD 80 and the voice encoder 82 are integrated into some speechcompression schemes such as those promulgated in ITU RecommendationsG.729 with Annex B VAD (March 1996)—Coding of Speech at 8 kbits/s UsingConjugate-Structure Algebraic-Code-Exited Linear Prediction (CS-ACELP),and G.723.1 with Annex A VAD (March 1996)—Dual Rate Coder for MultimediaCommunications Transmitting at 5.3 and 6.3 kbit/s, the contents of whichis hereby incorporated herein by reference as though set forth in fullherein.

[0052] Operating under the packet tone exchange, a DTMF detector 76determines whether or not there is a DTMF signal present at the nearend. The DTMF detector 76 also provides a pre-detection flag 76 a whichindicates whether or not it is likely that the digital voice samplemight be a portion of a DTMF signal. If so, the pre-detection flag 76 ais relayed to the packetization engine 78 instructing it to beginholding voice packets. If the DTMF detector 76 ultimately detects a DTMFsignal, the voice packets are discarded, and the DTMF signal is coupledto the packetization engine 78. Otherwise the voice packets areultimately released from the packetization engine 78 to the host (notshown). The benefit of this method is that there is only a temporaryimpact on voice packet delay when a DTMF signal is pre-detected inerror, and not a constant buffering delay. Whether voice packets areheld while the pre-detection flag 76 a is active could be adaptivelycontrolled by the user application layer.

[0053] Similarly, a call progress tone detector 77 also operates underthe packet tone exchange to determine whether a precise signaling toneis present at the near end. Call progress tones are those which indicatewhat is happening to dialed phone calls. Conditions like busy line,ringing called party, bad number, and others each have distinctive tonefrequencies and cadences assigned them. The call progress tone detector77 monitors the call progress state, and forwards a call progress tonesignal to the packetization engine to be packetized and transmittedacross the packet based network. The call progress tone detector mayalso provide information regarding the near end hook status which isrelevant to the signal processing tasks. If the hook status is on hook,the VAD should preferably mark all frames as inactive, DTMF detectionshould be disabled, and SID packets should only be transferred if theyare required to keep the connection alive.

[0054] The decoding system of the network VHD 62 essentially performsthe inverse operation of the encoding system. The decoding system of thenetwork VHD 62 comprises a de-packetizing engine 84, a voice queue 86, aDTMF queue 88, a precision tone queue 87, a voice synchronizer 90, aDTMF synchronizer 102, a precision tone synchronizer 103, a voicedecoder 96, a VAD 98, a comfort noise estimator 100, a comfort noisegenerator 92, a lost packet recovery engine 94, a tone generator 104,and a precision tone generator 105.

[0055] The de-packetizing engine 84 identifies the type of packetsreceived from the host (i.e., voice packet, DTMF packet, call progresstone packet, SID packet), transforms them into frames which are protocolindependent. The de-packetizing engine 84 then transfers the voiceframes (or voice parameters in the case of SID packets) into the voicequeue 86, transfers the DTMF frames into the DTMF queue 88 and transfersthe call progress tones into the call progress tone queue 87. In thismanner, the remaining tasks are, by and large, protocol independent.

[0056] A jitter buffer is utilized to compensate for network impairmentssuch as delay jitter caused by packets not arriving with the samerelative timing in which they were transmitted. In addition, the jitterbuffer compensates for lost packets that occur on occasion when thenetwork is heavily congested. In the described exemplary embodiment, thejitter buffer for voice includes a voice synchronizer 90 that operatesin conjunction with a voice queue 86 to provide an isochronous stream ofvoice frames to the voice decoder 96.

[0057] Sequence numbers embedded into the voice packets at the far endcan be used to detect lost packets, packets arriving out of order, andshort silence periods. The voice synchronizer 90 can analyze thesequence numbers, enabling the comfort noise generator 92 during shortsilence periods and performing voice frame repeats via the lost packetrecovery engine 94 when voice packets are lost. SID packets can also beused as an indicator of silent periods causing the voice synchronizer 90to enable the comfort noise generator 92. Otherwise, during far-endactive speech, the voice synchronizer 90 couples voice frames from thevoice queue 86 in an isochronous stream to the voice decoder 96. Thevoice decoder 96 decodes the voice frames into digital voice samplessuitable for transmission on a circuit switched network, such as a 64kb/s PCM signal for a PSTN line. The output of the voice decoder 96 (orthe comfort noise generator 92 or lost packet recovery engine 94 ifenabled) is written into a media queue 106 for transmission to the PXD60.

[0058] The comfort noise generator 92 provides background noise to thenear-end user during silent periods. If the protocol supports SIDpackets, (and these are supported for VTOA, FRF-11, and VoIP), thecomfort noise estimator at the far-end encoding system should transmitSID packets. Then, the background noise can be reconstructed by thenear-end comfort noise generator 92 from the voice parameters in the SIDpackets buffered in the voice queue 86. However, for some protocols,namely, FRF-11, the SID packets are optional, and other far-end usersmay not support SID packets at all. In these systems, the voicesynchronizer 90 continues to operate properly. In the absence of SIDpackets, the voice parameters of the background noise at the far end canbe determined by running the VAD 98 at the voice decoder 96 in serieswith a comfort noise estimator 100.

[0059] Preferably, the voice synchronizer 90 is not dependent uponsequence numbers embedded in the voice packet. The voice synchronizer 90can invoke a number of mechanisms to compensate for delay jitter inthese systems. For example, the voice synchronizer 90 can assume thatthe voice queue 86 is in an underflow condition due to excess jitter andperform packet repeats by enabling the lost frame recovery engine 94.Alternatively, the VAD 98 at the voice decoder 96 can be used toestimate whether or not the underflow of the voice queue 86 was due tothe onset of a silence period or due to packet loss. In this instance,the spectrum and/or the energy of the digital voice samples can beestimated and the result 98 a fed back to the voice synchronizer 90. Thevoice synchronizer 90 can then invoke the lost packet recovery engine 94during voice packet losses and the comfort noise generator 92 duringsilent periods.

[0060] When DTMF packets arrive, they are de-packetized by thede-packetizing engine 84. DTMF frames at the output of thede-packetizing engine 84 are written into the DTMF queue 88. The DTMFsynchronizer 102 couples the DTMF frames from the DTMF queue 88 to thetone generator 104. Much like the voice synchronizer, the DTMFsynchronizer 102 is employed to provide an isochronous stream of DTMFframes to the tone generator 104. Generally speaking, when DTMF packetsare being transferred, voice frames should be suppressed. To someextent, this is protocol dependent. However, the capability to flush thevoice queue 86 to ensure that the voice frames do not interfere withDTMF generation is desirable. Essentially, old voice frames which may bequeued are discarded when DTMF packets arrive. This will ensure thatthere is a significant gap before DTMF tones are generated. This isachieved by a “tone present” message 88 a passed between the DTMF queueand the voice synchronizer 90.

[0061] The tone generator 104 converts the DTMF signals into a DTMF tonesuitable for a standard digital or analog telephone. The tone generator104 overwrites the media queue 106 to prevent leakage through the voicepath and to ensure that the DTMF tones are not too noisy.

[0062] There is also a possibility that DTMF tone may be fed back as anecho into the DTMF detector 76. To prevent false detection, the DTMFdetector 76 can be disabled entirely (or disabled only for the digitbeing generated) during DTMF tone generation. This is achieved by a“tone on” message 104 a passed between the tone generator 104 and theDTMF detector 76. Alternatively, the NLP 72 can be activated whilegenerating DTMF tones.

[0063] When call progress tone packets arrive, they are de-packetized bythe de-packetizing engine 84. Call progress tone frames at the output ofthe de-packetizing engine 84 are written into the call progress tonequeue 87. The call progress tone synchronizer 103 couples the callprogress tone frames from the call progress tone queue 87 to a callprogress tone generator 105. Much like the DTMF synchronizer, the callprogress tone synchronizer 103 is employed to provide an isochronousstream of call progress tone frames to the call progress tone generator105. And much like the DTMF tone generator, when call progress tonepackets are being transferred, voice frames should be suppressed. Tosome extent, this is protocol dependent. However, the capability toflush the voice queue 86 to ensure that the voice frames do notinterfere with call progress tone generation is desirable. Essentially,old voice frames which may be queued are discarded when call progresstone packets arrive to ensure that there is a significant inter-digitgap before call progress tones are generated. This is achieved by a“tone present” message 87 a passed between the call progress tone queue87 and the voice synchronizer 90.

[0064] The call progress tone generator 105 converts the call progresstone signals into a call progress tone suitable for a standard digitalor analog telephone. The call progress tone generator 105 overwrites themedia queue 106 to prevent leakage through the voice path and to ensurethat the call progress tones are not too noisy.

[0065] The outgoing PCM signal in the media queue 106 is coupled to thePXD 60 via the switchboard 32′. The outgoing PCM signal is coupled to anamplifier 108 before being outputted on the PCM output line 60 b.

[0066] Referring again to FIG. 2, the low-band data, L and L1, isdesigned to have sufficient separation from the high-band data, H andH1. This is because narrowband services will, in one embodiment, operateonly on the low-band data and not on the high-band data. In anillustrative embodiment of the present invention, the band splitters230, 264 and band combiners 250, 272 are substantially lossless.

[0067] In the illustrative embodiment shown in FIG. 2, the band splitter230 and band combiner 272 reside with PXD 225, and band splitter 264 andband combiner 250 reside with PVE 255. This implementation may bepreferred because the PXD 225 and PVE 255 understand the band-split datastreams and need to operate on both band-split and band-combined data.In an alternative embodiment of the present invention, the band splitter230 and band combiner 272 may reside external to PXD 225, and bandsplitter 264 and band combiner 250 reside external to PVE 255. 1681 FIG.4 is a block diagram of a band splitter 400 such as band splitters 230and 264 of FIG. 2, according to an illustrative embodiment of thepresent invention. Band splitter 400 comprises finite impulse response(FIR) filter h 410, down-sampler 420, up-sampler 430, FIR filter g 440,and delay element 450. The wideband data 405 is filtered by FIR filter h410 and the filter output is then down-sampled by a factor of two bydown-sampler 420 to produce the low-band data 470. To obtain thehigh-band data 480, the low-band data 470 is up-sampled by a factor oftwo by up-sampler 430, and the up-sampled data is filtered by FIR filterg 440. The wideband data 405 is delayed by delay element 450. The delay,D, of delay element 450 is the filter order. To remove the spectralcontent of the low-band data from the delayed wideband data, the outputof FIR filter g 440 is subtracted from the delayed wideband data toobtain the high-band data 480. The removal of the spectral content ofthe low-band data from the wideband data can be accomplished as in theembodiment illustrated in FIG. 4, or in another embodiment, for example,by inverting the output of FIR filter g 440 and adding the invertedlow-band data to the delayed wideband data, by negating the coefficientsof FIR filter g 440, or by delaying the low-band data by a non-causalamount either explicitly or via a non-causal filter. Other methods ofremoving the spectral content of the low-band data from the widebanddata may also be used without departing from the spirit of the presentinvention.

[0068]FIG. 5 is a block diagram of a band combiner 500 such as bandcombiners 250 and 272 of FIG. 2, according to an illustrative embodimentof the present invention. Band combiner 500 comprises up-sampler 510,FIR filter g 520, and adder 530. Low-band data 505 is up-sampled by afactor of two by up-sampler 510 and the up-sampler output is filtered byFIR filter g 520. The output of filter g 520 is added to the high-banddata 515 at adder 530 to produce the wideband data 540. The combiner isessentially defined by the band splitter definition; i.e., theup-sampled low-band data is filtered through filter g 520 and added tothe high-band data 515.

[0069] Band splitter 400 and band combiner 500 are lossless independentof the filters h 410 and g 440, 520. The filters are designed such thatthe high-band data really is high band. If a requirement exists that thespectral mask of the low-band data meet the spectral mask of G.712, h isnon-linear phase. This requirement may also impose some limitations onthe attenuation at about 4 kHz.

[0070] The filter h 410 is illustratively an FIR filter (of length N)having a frequency response that is maximally flat in the pass-band.Such a frequency response may be achieved, for example, via theParks/McClellan/Remez exchange algorithm. The filter illustrativelymeets G.712 frequency response specifications. All roots are reflectedinside the unit circle to obtain a minimum phase filter. The frequencyresponse of filter h 410 may be defined as H_(o)(z).

[0071] The filter g 440, 520 is then defined by a frequency responseG_(o)(z)=z^(−(N−1))H_(o)(z⁻¹). Thus, all coefficients in H_(o) arereversed. In other words, G_(o)(z) is maximum phase. Since G_(o) isH_(o) time-reversed, the low-band data 470 is linear phase with delay N.Note that H_(o)(z) is not linear phase, but is maximum phase. Thus, ifthe input data 405 were filtered through H_(o)(z), then down-sampled,up-sampled, and passed through H_(o)(z) again, it would not be linearphase (although it would be minimum phase). If a signal is filteredthrough a filter h=h(0) . . . h(N−1) (which is non-symmetric) and thenfiltered through h reversed (i.e., hr=h(N−1 . . . h(0)) then the overallend-to-end result is linear phase.

[0072] In an alternative embodiment of the present invention, G_(o)(z)is equal to H_(o)(z) and H_(l)(z)=z^(−L)−H_(o)(z)H₀(z). In thisembodiment, H_(o)(z) is high-pass. This achieves near-perfectreconstruction.

[0073] In another alternative embodiment, G_(o)(z) is equal to H_(o)(z)and H_(l)(z)=A(z)−H_(o)(z)H_(o)(z), where A(z) is an all-pass function.This gives near-perfect magnitude reconstruction but with some phase“error.”

[0074] In yet a further embodiment wherein H_(o)(z) is used for thereconstruction filter, H_(o)(z) has the same magnitude response asG_(o)(z). In this case we can write G_(o)(z)=H_(o)(z) A_(o)(z), whereA_(o)(z) is an all-pass function given by A(z)=G_(o)(z)/H_(o)(z). Thehigh-pass branch is filtered via l/A_(o)(z) (which is still anall-pass). In one such embodiment, the reconstruction filter is forcedto be linear phase. If this is accomplished, the filter bank is notperfect reconstruction, but it is minimum phase (with no magnitudedistortion). There is a MIPs penalty in running the all-pass, and thereis no guarantee it will be well-behaved (although it should be). Theresult is that this gives a tradeoff between delay (minimum phase) andlinearity of phase.

[0075] As describe previously, in the exemplary embodiment of FIG. 4 thereconstructed low-band data is removed from the wideband data 405delayed by N using subtraction, to give the high-band data 480. Bydefinition, this is perfect reconstruction. Since the overall transfercharacteristic of the low-band filter is linear phase, the transferfunction to obtain the high-band data is “high-pass”, assuming thelow-band filter is maximally flat in the pass-band.

[0076] The band combiner 500 re-samples the low-band data 505 with theup-sampler 510 and filter g 520, and then adds in the high-band data515, producing wideband data 540.

[0077] If the low-band data (8 kHz sampling) is used in, for example,the switchboard 210 of FIG. 2 to conference with a narrowband coder,then G.712 is met. If the wideband data is used, it is perfectreconstruction. The wideband data has a delay of N and is linearphase/perfect reconstruction. In illustrative embodiments, a delay N of32 or 40 is used, which would be a 2 or 2.5 milliseconds (ingress) delayat 16 kHz sampling.

[0078] The above-described filter design achieves G.712 compliance,G.722 compliance, good separation (little overlap) of the low and highbands, and substantially perfect reconstruction of the signals.

[0079] Referring now to FIG. 6, there is illustrated a block diagram ofan exemplary terminal 658, corresponding to terminal 110 as depicted inFIG. 1, in which an embodiment according to the present invention may bepracticed. A processor 660 is interconnected via system bus 662 torandom access memory (RAM) 664, read only memory (ROM) 666, aninput/output adapter 668, a user interface adapter 672, a communicationsadapter 684, and a display adapter 686. The input/output adapter 668connects peripheral devices such as hard disc drive 640, floppy discdrives 641 for reading removable floppy discs 642, and optical discdrives 643 for reading removable optical disc 644. The user interfaceadapter 672 connects devices such as a keyboard 674, a speaker 678, andmicrophone 682 to the bus 662. The microphone 682 generates audiosignals which are digitized by the user interface adapter 672. Thespeaker 678 receives audio signals which are converted from digitalsamples to analog signals by the user interface adapter 672. The displayadapter 686 connects a display 688 to the bus 662. It will be clear toone skilled in the art that embodiments of the present invention mayalso be practiced in other types of terminals as well, including but notlimited to, a telephone without a hard disk drive 640, a floppy diskdrive 641, nor optical disk drive 643, in which the program instructionsmay be stored in ROM 666, or downloaded over communications adapter 684and stored in RAM 664. An embodiment may also be practiced in, forexample, a portable hand-held terminal with little or no displaycapability, in a consumer home entertainment system, or even in amulti-media game system console.

[0080] An embodiment of the present invention can be implemented as setsof instructions resident in the RAM 664 or ROM 666 of one or moreterminals 658 configured generally as described in FIG. 6. Untilrequired by the terminal 658, the set of instructions may be stored inanother memory readable by the processor 660, such as hard disc drive640, floppy disc 642, or optical disc 644. One skilled in the art wouldappreciate that the physical storage of the sets of instructionsphysically changes the medium upon which it is stored electrically,magnetically, or chemically so that the medium carries informationreadable by a processor.

[0081] Accordingly, the present invention may be realized in hardware,software, or a combination of hardware and software. The presentinvention may be realized in a centralized fashion in one computersystem, or in a distributed fashion where different elements are spreadacross several interconnected computer systems. Any kind of computersystem or other apparatus adapted for carrying out the methods describedherein is suited. A typical combination of hardware and software may bea general-purpose computer system with a computer program that, whenbeing loaded and executed, controls the computer system such that itcarries out the methods described herein.

[0082] The present invention also may be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext means any expression, in any language, code or notation, of aset of instructions intended to cause a system having an informationprocessing capability to perform a particular function either directlyor after either or both of the following: a) conversion to anotherlanguage, code or notation; b) reproduction in a different materialform.

[0083] Notwithstanding, the invention and its inventive arrangementsdisclosed herein may be embodied in other forms without departing fromthe spirit or essential attributes thereof. Accordingly, referenceshould be made to the following claims, rather than to the foregoingspecification, as indicating the scope of the invention. In this regard,the description above is intended by way of example only and is notintended to limit the present invention in any way, except as set forthin the following claims.

[0084] While the present invention has been described with reference tocertain embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the scope of the present invention. In addition,many modifications may be made to adapt a particular situation ormaterial to the teachings of the present invention without departingfrom its scope. Therefore, it is intended that the present invention notbe limited to the particular embodiment disclosed, but that the presentinvention will include all embodiments falling within the scope of theappended claims.

What is claimed is:
 1. A band splitter comprising: at least one filterfor modifying wideband data, producing low-band data; a delay elementfor delaying the wideband data; and a device for removing the low-banddata from the delayed wideband data, producing high-band data.
 2. Theband splitter of claim 1, wherein the wideband data represents spectralcomponents from a predetermined lower frequency to a predetermined upperfrequency.
 3. The band splitter of claim 1 wherein the predeterminedlower frequency is approximately 0 Hz.
 4. The band splitter of claim 2,wherein the predetermined upper frequency is approximately 8 kHz.
 5. Theband splitter of claim 1, wherein the low-band data represents spectralcomponents less than a predetermined frequency, and the high-band datarepresents spectral components greater than the predetermined frequency.6. The band splitter of claim 5, wherein the predetermined frequency isapproximately 4 kHz.
 7. The band splitter of claim 1, wherein thespectral mask of the low-band data meets the spectral mask of G.712. 8.The band splitter of claim 1, wherein the at least one filter furthercomprises a down sampler for reducing the sampling rate of the low-banddata.
 9. A band splitter comprising: a first filter for filteringwideband data; a down-sampler for down-sampling the output of the firstfilter, producing low-band data; an up-sampler for up-sampling thelow-band data; a second filter for filtering the up-sampled low-banddata; a delay element to delay the wideband data; and a device forremoving the output of the second filter from the delayed wideband data,producing high-band data.
 10. The band splitter of claim 9, wherein thewideband data represents spectral components from a predetermined lowerfrequency to a predetermined upper frequency.
 11. The band splitter ofclaim 10, wherein the predetermined lower frequency is approximately 0Hz.
 12. The band splitter of claim 10, wherein the predetermined upperfrequency is approximately 8 kHz.
 13. The band splitter of claim 9,wherein the low-band data represents spectral components less than apredetermined frequency, and the high-band data represents spectralcomponents greater than the predetermined frequency.
 14. The bandsplitter of claim 13, wherein the predetermined frequency isapproximately 4 kHz.
 15. The band splitter of claim 9, wherein thespectral mask of the low-band data meets the spectral mask of G.712. 16.A band combiner comprising: an up-sampler to up-sample low-band data; afilter for filtering the output of the up-sampler; and an adder forcombining the output of the filter and high-band data, producingwideband data.
 17. The band combiner of claim 16, wherein the widebanddata represents spectral components from a predetermined lower frequencyto a predetermined upper frequency.
 18. The band combiner of claim 17,wherein the predetermined lower frequency is approximately 0 Hz.
 19. Theband combiner of claim 17 wherein the predetermined upper frequency isapproximately 8 kHz.
 20. The band combiner of claim 16, wherein thespectral mask of the low-band data meets the spectral mask of G.712. 21.A method of splitting wideband data into low-band data and high-banddata, the method comprising: filtering the wideband data to producelow-band data; delaying the wideband data; and removing the low-banddata from the delayed wideband data to produce high-band data.
 22. Themethod of claim 21, wherein the wideband data represents spectralcomponents from a predetermined lower frequency to a predetermined upperfrequency.
 23. The method of claim 22, wherein the predetermined lowerfrequency is approximately 0 Hz.
 24. The method of claim 22, wherein thepredetermined upper frequency is approximately 8 kHz.
 25. The method ofclaim 21, wherein the low-band data represents spectral components lessthan a predetermined frequency, and the high-band data representsspectral components greater than the predetermined frequency.
 26. Themethod of claim 25, wherein the predetermined frequency is approximately4 kHz.
 27. The band splitter of claim 21, wherein the spectral mask ofthe low-band data meets the spectral mask of G.712.
 28. A method ofsplitting wideband data into low-band data and high-band data, themethod comprising: filtering the wideband data; down-sampling thefiltered wideband data to produce low-band data; up-sampling thelow-band data; filtering the up-sampled low-band data; delaying thewideband data; and removing the filtered up-sampled low-band data fromthe delayed wideband data to produce high-band data.
 29. The method ofclaim 28, wherein the wideband data represents spectral components froma predetermined lower frequency to a predetermined upper frequency. 30.The method of claim 29, wherein the predetermined lower frequency isapproximately 0 Hz.
 31. The method of claim 29, wherein thepredetermined upper frequency is approximately 8 kHz.
 32. The method ofclaim 28, wherein the low-band data represents spectral components lessthan a predetermined frequency, and the high-band data representsspectral components greater than the predetermined frequency.
 33. Themethod of claim 32, wherein the predetermined frequency is approximately4 kHz.
 34. The band splitter of claim 28, wherein the spectral mask ofthe low-band data meets the spectral mask of G.712.
 35. A method ofcombining low-band data and high-band data to produce wideband data, themethod comprising: up-sampling the low-band data; filtering theup-sampled low-band data; and adding the filtered up-sampled low-banddata to the high-band data to produce wideband data.
 36. The method ofclaim 35, wherein the wideband data represents spectral components froma predetermined lower frequency to a predetermined upper frequency. 37.The method of claim 36, wherein the predetermined lower frequency isapproximately 0 Hz.
 38. The method of claim 36, wherein thepredetermined upper frequency is approximately 8 kHz.
 39. The method ofclaim 35, wherein the low-band data represents spectral components lessthan a predetermined frequency, and the high-band data representsspectral components greater the predetermined frequency.
 40. The methodof claim 39, wherein the predetermined frequency is approximately 4 kHz.41. The method of claim 35, wherein the spectral mask of the low-banddata meets the spectral mask of G.712.
 42. A machine-readable storage,having stored thereon a computer program having a plurality of codesections for implementing a band splitter, the code sections executableby a machine for causing the machine to perform the operationscomprising: filtering wideband data; down-sampling the filtered widebanddata to produce low-band data; up-sampling the low-band data; filteringthe up-sampled low-band data; delaying the wideband data; and removingthe filtered up-sampled low-band data from the delayed wideband data toproduce high-band data.
 43. The method of claim 42, wherein the widebanddata represent spectral components from a predetermined lower frequencyto a predetermined upper frequency.
 44. The method of claim 43, whereinthe predetermined lower frequency is approximately 0 Hz.
 45. The methodof claim 43, wherein the predetermined upper frequency is approximately8 kHz.
 46. The method of claim 42, wherein the low-band data representsspectral components less than a predetermined frequency, and thehigh-band data represents spectral components greater than thepredetermined frequency.
 47. The method of claim 46, wherein thepredetermined frequency is approximately 4 kHz.
 48. The method of claim42, wherein the spectral mask of the low-band data meets the spectralmask of G.712.
 49. A machine-readable storage, having stored thereon acomputer program having a plurality of code sections for implementing aband combiner, the code sections executable by a machine for causing themachine to perform the operations comprising: up-sampling low-band data;filtering the up-sampled low-band data; and adding the filteredup-sampled low-band data to high-band data to produce wideband data. 50.The method of claim 49, wherein the wideband data represents spectralcomponents from a predetermined lower frequency to a predetermined upperfrequency.
 51. The method of claim 50, wherein the predetermined lowerfrequency is approximately 0 Hz.
 52. The method of claim 50, wherein thepredetermined upper frequency is approximately 8 kHz.
 53. The method ofclaim 49, wherein the low-band data represents spectral components lessthan a predetermined frequency, and the high-band data representsspectral components greater than the predetermined frequency.
 54. Themethod of claim 53, wherein the predetermined frequency is approximately4 kHz.
 55. The method of claim 49, wherein the spectral mask of thelow-band data meets the spectral mask of G.712.